The invention relates to a method for analysis of a sample which includes:
a substrate containing a chemical element b,
a thin layer which is deposited on the substrate and contains the same chemical element b and a chemical element a, the method including the steps of:
irradiating the sample by means of primary X-rays,
detecting the intensity of X-ray lines of comparatively hard fluorescent X-rays and of comparatively soft fluorescent X-rays, both generated in the sample due to the irradiation by means of the primary X-rays.
The invention also relates to a device for carrying out the above method.
A method and a device of this kind are known from Japanese patent No. 2706601. The cited Japanese patent describes a method for determining the thickness of a thin layer on a substrate and for determining the concentration of a chemical element in said thin layer by means of fluorescent X-ray analysis. To this end, the sample is irradiated by means of X-rays (the primary radiation) originating from an X-ray source. The sample is formed by a substrate which consists of a first chemical element, in this case being silicon, on which there is provided a thin layer which consists of the first chemical element (silicon) and a second chemical element, in this case being tungsten. The primary radiation generates fluorescent X-rays in the sample, which fluorescent X-rays contain characteristic radiation of the chemical elements present in the sample, so silicon and tungsten. This characteristic radiation includes comparatively hard X-rays (for example, having a wavelength of the order of magnitude of from 0.02 to 1 nm) as well as comparatively soft X-rays (for example, having a wavelength of the order of magnitude of from 2 to 15 nm).
According to the known method the apparatus response is determined by means of the known reference samples. Using such an apparatus response, the layer thickness and the concentration of one of the chemical elements in the layer, for example tungsten, are determined on the basis of the measured intensities of both the comparatively hard and the comparatively soft fluorescent radiation. Such a determination is performed by means of a known method for the theoretical calculation of the intensity of fluorescent X-rays; this calculation method is known as the xe2x80x9cfundamental parameter methodxe2x80x9d. Such a theoretically calculated intensity of fluorescent X-rays is the intensity occurring immediately after departure from the sample; therefore, the effect of the entire measuring channel from the sample up to and including the detector is not taken into account therein. In the cited Japanese patent the combination of the two types of fluorescent X-rays (i.e. the comparatively hard and the comparatively soft fluorescent rays) is formed by one of the combinations of X-ray lines K with M, K with N and L with N of tungsten or one of the combinations of X-ray lines K with M, K with N and L with N of silicon.
The known method, however, has the drawback that only a limited number of combinations of X-ray lines is used for the analysis of the sample, so that the possibility for analysis are severely restricted. For example, a thin layer of cobalt silicide is not covered by said method.
It is an object of the present invention to provide a method and a device wherein said drawback is mitigated.
To this end, according to the invention one or more of the following combinations of X-ray lines of the chemical elements a and b are detected:
b-L/b-K;
b-K/a-L;
b-L/a-L;
b-L/a-M;
a-M/a-L;
a-M/a-N;
a-K/b-K;
a-K/b-L;
a-K/a-L;
and the thickness of the thin layer and/or the concentration of the first or the second chemical element is determined on the basis of the X-ray lines determined for the comparatively hard and for the comparatively soft fluorescent X-rays.
In conformity with a further aspect of the invention there is provided a device for carrying out the above-mentioned method, including:
a sample holder for receiving the sample to be analyzed;
an X-ray source for producing the primary X-rays;
selection means for the selection at option of at least the comparatively hard and the comparatively soft X-rays;
detection means for detecting the intensity of the X-ray lines of the fluorescent X-rays emitted by the sample;
wherein the selection means include a first selector which is arranged in a first channel so as to select the comparatively soft fluorescent X-rays and a second selector which is arranged in a second channel so as to select the comparatively hard fluorescent X-rays; and
the detection means include a first detector which is arranged in a first channel and a second detector which is arranged in a second channel, said detectors simultaneously detecting the comparatively hard and the comparatively soft fluorescent X-rays.
Because of the simultaneous detection of the comparatively hard and the comparatively soft fluorescent X-rays in separate channels, the period of time required for detection can be reduced. Moreover, the selection means, detection means and focusing means, if any, can be selected so as to optimize the detection sensitivity in each channel.
In a further preferred embodiment, the pressure in each of the channels is less than 10xe2x88x926 bar, thus enabling the analysis of comparatively light elements.